perl -ne '$lines++;$common++ if ($seen{$_} .= @ARGV) =~ /10$/; END{printf("%.2f",$common/$lines*200)}' file1.txt file2.txt

##</code><code>##

perl -ne '$lines++;$common++ if ($seen{$_} .= @ARGV) eq "10"; END{printf("%.2f",$common/$lines*200)}' file1.txt file2.txt